ISO 100 and ISO 12800 cannot be set (EOS REBEL T3i/EOS 600D)

Answer

If you set [C.Fn II -6: Highlight tone priority] to [1:Enable] on your camera, the settable ISO range will be limited to ISO 200-6400, so ISO 100 or ISO 12800 cannot be selected. The [D+] icon will be displayed on the LCD panel and in the viewfinder when highlight tone priority is enabled.

If you wish to make the ISO 100 or ISO 12800 setting (ISO expansion), change the setting as follows.